Rechargeable batteries have received tremendous attention in recent years. Such batteries also have come to be known as “secondary batteries” or even as “storage batteries”. They can be operated to store a charge, and thereafter operated to discharge the charge to provide a source of electricity to a device. In general, these type of batteries have a small number of active components, which include the electrodes (specifically the anode and the cathode), which cooperate together to perform a reversible electrochemical reaction. In general, efforts to improve durability and efficiency of rechargeable batteries have concentrated in many instances upon the improvement of one of more of these active components.
One increasingly popular type of battery is a battery that employs a metal ion (e.g., a lithium-ion) in a generally cohesive mass of an electrolyte material. When an electrochemical cell of such a battery is discharging, generally lithium ions extracted from the anode flow to the cathode. When the cell is charging, the reverse process occurs. Lithium ions become extracted from the cathode and flow to and become inserted into the anode.
As indicated, the generally cohesive mass of electrolyte material is regarded as a solid electrolyte, even though some such solid electrolytes have gel characteristics. The metal ions (e.g., lithium ions) are present in a sufficient concentration, and are of such a size that as the ions flow between electrodes, the electrolyte and other active components may be actually susceptible to dimensional fluctuations as a result of the ion flow. Thus, as ions flow into a region, they will swell that region, and the region from which the ions flowed will shrink. Dimensional fluctuations may also arise as a result of heat build-up in the electrolyte due to the energy produced by the electrochemical reaction. In the design of active components for rechargeable batteries, it is therefore important that the materials employed for the active components be capable of withstanding the dynamic cyclical dimensional fluctuations.
As can be appreciated, materials suitable as electrodes for rechargeable batteries often require an appropriate balance of mechanical properties and electrical properties, which balance does not usually exist in a single homogeneous material. Thus, to achieve a suitable balance of properties it has been proposed to employ composite materials; that is, to employ materials that include two or more chemically and/or physically different constituent materials that are combined into a single material. The constituent materials, though forming a single composite material, will result in generally two or more discrete phases. The constituent materials effectively remain as separate and distinct materials within the composite. By way of example, particles of an active conductive material (referred to herein as electroactive particles or “EAPs”) may be dispersed in a suitable binder material, such as a matrix formed of polymeric materials. The EAPs help provide desired electrical characteristics and the binder helps to impart appropriate mechanical or other properties. To this combination of materials it is also possible that one of more electrode materials (e.g., intercalation compounds) may be mixed. An example of a composite is illustrated in U.S. Pat. No. 6,455,194, in which a phenol-formaldehyde material is used in a binder, alone or with polyvinylidene fluoride (PVDF). See also, U.S. Pat. Nos. 6,855,273 and 6,174,623.
One possible approach to reduce the burden upon EAPs for delivering electrical characteristics has been to employ a conductive polymer such as PVDF in the binder. Unfortunately, existing conductive polymers have limited ability to combine with EAPs, because of their brittle characteristics, their high melting temperature, or both. Interfacial bods between the EAPs and the polymer tend to be relatively weak. In some instances, especially as a result of the cyclical dimensional fluctuations discussed above, the binder polymer and the EAPs may experience an undesirable degree of separation or “pull-out” in service, which has the potential to compromise battery performance. Long term durability issues thus may become a problem. Also, the brittle characteristic of polymers such as PVDF cannot be easily bent and folded which additionally limits the form factor of batteries including these materials.
Certain ethylene oxide-containing homopolymer or copolymer materials may exhibit attractive electrical characteristics for use as an active rechargeable battery component, such as in a binder of an electrode or in an electrolyte. However, these materials generally have been avoided because of synthesis constraints, the difficulties in achieving the necessary mechanical characteristics of the materials to effectively fulfill the processing and/or mechanical needs of the material in active component applications.
It would be particularly attractive to achieve a durable and long-lasting composite material that has utility as an electrode for a rechargeable battery, particularly one that has a strong adhesion to electroactive particles, even after undergoing mechanical deformation, such as the dynamic cyclical dimensional fluctuations typical during the cyclic charging of a battery. Additionally, it would be attractive to achieve a more flexible electrode which may improve the form factor (e.g., by the ability for free folding the electrode), be easier to process into a battery (and thus increase production yields), and eliminate the need for the relatively stiff packaging.